Trace Elements in Magnetite and Origin of the Mariela Iron Oxide-Apatite Deposit, Southern Peru

Abstract

To better understand the origin of the Andean iron oxide-apatite (IOA) deposits, we conducted a study on the geology and magnetite geochemistry of the Mariela IOA deposit in the Peruvian Iron Belt, central Andes. The Mariela deposit is hosted by gabbroic and dioritic intrusions. The major high-grade massive ores are primarily composed of magnetite and contain variable amounts of apatite and actinolite. Based on textural and geochemical characteristics, three different types of magnetite are recognized: Type I magnetite occurs in the massive magnetite ore, subclassified as inclusion-rich (I-a), inclusion-free (I-b), and mosaic (I-c); Type II magnetite is associated with abundant actinolite and titanite; and Type III magnetite is disseminated in altered host rocks. However, the magnetite geochemistry data for the Mariela deposit plot shows different genetic areas in [Ti + V] vs. [Al + Mn], Ti vs. V, and Fe vs. V/Ti discrimination diagrams, indicating a paradox of magmatic and hydrothermal origins. Our interpretation is as follows: Type I-a magnetite had an initial magmatic or high-temperature magmatic-hydrothermal origin, with slight modifications during transportation and subsequent hydrothermal precipitation (Types I-b and I-c). Type II magnetite is formed from hydrothermal fluid due to the presence of abundant actinolite. Disseminated magnetite (Type III) and veinlet-type magnetite formed after fluid replacement of the host rock. We stress that elemental discrimination diagrams should be combined with field studies and textural observations to provide a reasonable geological interpretation. A clear cooling trend is evident among the three subtypes of Type I magnetite (I-a, I-b, and I-c), as well as Type II and Type III magnetite, with average formative temperatures of 737 °C, 707 °C, 666 °C, 566 °C, and 493 °C, respectively. The microanalytical data on magnetite presented here support the magmatic-hydrothermal flotation model to explain the origin of IOA deposits in the Coastal Cordillera of Southern Peru.